What is Color?

"Color" is the name we assign to our internal representation of the spectrum of wavelengths reflected by objects. Humans gain color vision from having three types of cone receptors. These receptors respond to different wavelengths and have broadly overlapping response curves, so that some of our color perception results from the combined effects of two receptors responding to an intermediate wavelength. Human eyes respond to wavelengths from blue to red; our eyes exclude ultraviolet and infrared information.

Colors, though, may be non-spectral. We assign the name "purple" to our perception of the combination of our red and blue receptors. No one wavelength produces purple.

Animal color perception can differ from humans in three important ways:

different numbers of color receptors
Some apes and monkeys share the human trait of having three types of color receptors--red, green and blue. Most mammals, though, have only two color receptor types, one in the blue-violet (or even in the ultraviolet) range and the other in the green. (So much, by the way, for red being a meaninful warning coloration for the bulk of the mammals.) Most birds and some fish (eg, goldfish) have four color receptor types. Some insects have five or even more receptor types. Having multiple color receptors creates more possibilities for non-spectral colors (analogous to purple). Four receptor types translates into a mind-boggling four non-spectral colors, illustrating the difficulty of imagining the visual world of other species.
different amount of overlap between the wavelength response ranges of receptors
Broadly overlapping receptor ranges, such as those foud in humans, result in the perception of intermediate colors. Intermediacy reduces contrast. More narrowly defined receptor ranges, such as the honey bee's, increase the perceived contrast between colors. Contrast may be critical for picking small items, such as flowers, out of monotonously colored backgrounds.
extensions of the range of perception into the ultraviolet or infrared range, or absence of perception in parts of our visual range
We often assume that animal's spectral sensitivity matches ours. However, there is a wealth of information available in ultraviolet and infrared, and many animals are able to extract that information from the environment. Bees see ultraviolet, but not red. Not surprisingly, flowers pollinated by bees are rarely red, but many have ultraviolet patterning that is invisible to the unaided human eye. Most birds can see near ultraviolet wavelengths, as well as the entire human visual spectrum.

The answer to the question "What is color", then, is difficult. The most important point is that humans should not assume that any animal species perceives the world in the same way as we do.

The spectral sensitivity of the European starling, showing this birds four color receptors and its visual responsiveness in the ultraviolet (redrawn after Cuthill et al. 2000).

Cuthill, I. C., J. C. Partridge, and A. T. D. Bennett. 2000. Avian UV vision and sexual selection. in Animal Signals, Y. Espmark, T. Amundson, and G. Rosenquist eds. Tapir Academic Press: Trondheim. pp 61-82.